Qatar Petroleum International (QPI) will pick up a 25 percent stake in the $4-billion Long Son petrochemical project in Vietnam, a senior executive of the southeast Asian country's state oil and gas group Petrovietnam said on Monday.

"We will establish a joint venture to build the complex," Do Van Hau, Petrovietnam chief executive officer said after signing an investment agreement with QPI officials.

Detailed work on the joint venture will start next month, the Hanoi-based group's sources said.

The complex will be Vietnam's first fully integrated petrochemical project, consisting of upstream, intermediate and downstream plants, QPI said in a separate statement.

The venture is made up of Petrovietnam, Vietnam's Vinachem, SCG chemicals and Thai Plastics and Chemicals Co Pcl, it said.

The announcement came during a visit to Vietnam of the Emir of Qatar, Sheikh Hamad bin Khalifa al-Thani.

Long Son refinery, part of project, 120 km (75 miles) southeast of Ho Chi Minh City, would use Middle Eastern crude oil, Petrovietnam has said.